1. Field of the Invention
This invention is directed to a field emitter device for use in flat panel displays, etc.
2. Description of Related Art
Electron field emitter structures have potential application in many different areas, for example to produce flat panel displays (Field Emitter Displays or FED's) and high frequency electronic devices. In each case the device would consist of an array of many individual field emitting cathodes. In most cases it is desirable that
i) electron emission occurs at low voltages, PA1 ii) high emission currents per unit area of the device can be achieved, and PA1 iii) the capacitance between the cathode and the gate electrode is kept to a minimum. PA1 iv) leakage between cathodes and gate layer is minimised. PA1 i) Each individual field emitting cathode that is in electrical contact with the gate electrode does not emit any signal. Since the number of pores in an anodic aluminium oxide film is large, at least about 10.sup.8 pores cm.sup.-2 it is possible to tolerate a situation in which a substantial number of individual field emitters is non functional and still get an acceptable signal. Nevertheless, if the problem of short circuits is not addressed, it can be found that more than 99%, e.g. more than 999 per thousand, individual field emitting cathodes are non-functional. PA1 ii) To obtain field emission from these devices, a substantial voltage is applied between the gate electrode on one surface of the anodic oxide layer and a conducting substrate constituting an address electrode overlying the other surface. These potentials may be of the order of volts or tens of volts. When there is a short circuit, through an individual emitter cathode, between the gate electrode and the address electrode, the current that passes may be sufficiently heavy to cause local damage to the device. PA1 iii) When there is a short circuit, through an individual field emitting cathode, between the gate electrode and address electrode, the leakage current between cathode and gate electrode is increased. PA1 iv) The leakage current between the gate electrode and the address electrode through the short circuit may be sufficiently large so as to reduce locally the voltage difference between gate and cathodes due to voltage drops caused by the large current flowing through the gate layer, address layer and the rest of the structure. This voltage drop may be sufficient to prevent other cathodes in the surrounding area emitting. PA1 a) providing a dielectric anodic metal oxide layer having a front surface and a back surface and an array of pores extending through the anodic metal oxide layer from the front surface to the back surface, PA1 b) providing wires in the pores having back ends and front ends to constitute individual field emitting cathodes, PA1 c) providing a gate electrode overlying the front surface of the anodic metal oxide layer, and PA1 d) providing an address electrode overlying the back surface of the anodic metal oxide layer and in electrical contact with the back ends of the wires, characterised by taking one or both of the following steps to reduce the extent of short circuits between the address electrode and the gate electrode:
The practical reasons for i) and ii) is that this determines the transconductance of the device and hence its maximum operating speed--an important parameter for high speed devices. In addition for FED's, operation at low voltages is attractive since this will lower the cost of the driving electronics used to control image generation. Whilst ii) is advantageous because if many emitters are fabricated per unit area this will tend to average the "flicker noise" that may be associated with each individual emitter so leading to a "cleaner" image.
The reason for iii) is that this also determines the maximum speed at which a device may work and also for relatively low frequency applications such as FED's driving the capacitance of the dielectric contributes substantially to the overall power consumption of the unit.
The reason for iv) is that leakage between the cathodes and gate layer increases the power consumption of the device and also makes uniform display brightness for a FED harder to achieve. This is because current flowing between cathode and gate will not contribute to exciting the display phosphors and thus those areas of a display having large leakage current will be less bright for a given total cathode current.
In order that factors i) to iv) may be realised an ideal field emitter geometry may be postulated. This would comprise i) a very small cathode to gate spacing so as to minimise the applied voltage required to produce field emission. ii) A very high density of field emitters per unit area each capable of delivering substantial current. iii) A thick dielectric separating the gate electrode from the substrate. iv) No electrical contact between the emitters and gate layer.
When aluminium is anodised in an electrolyte having some dissolving power for the oxide, there results a porous anodic aluminium oxide film which may be regarded as consisting of an array of hexagonal cells with a pore in the centre of each cell. When removed from the aluminium metal substrate on which it was formed, this anodic oxide film forms a potentially excellent dielectric layer for such field emitter structures. U.S. Pat. No. 5,164,632 describes an electron emitting element for use in a display device, in which the pores of an anodic aluminium oxide film are filled with metal and constitute electron emitting members, and a gate electrode disposed on one surface of the insulator and having protrusions which protrude into each of the pores. Although in principle the electron emitting members are not in electrical contact with the gate electrode, in practice short circuits are probable and no means is disclosed for dealing with the problem. U.S. Pat. No. 5,315,206 describes similar and related devices.
This invention is based on the realisation that short circuits between the gate electrode and individual field emitter electrodes is a major problem when using dielectric layers of anodic aluminium oxide, and one which must be addressed in order to produce a usable device. There are several related problems:
It is an object of this invention to provide field emitter structures in which these problems are minimised or overcome.